1. Field of the Invention
The present invention relates to a wiring board and a circuit apparatus using the same including a signal line and a capacitive element for transmitting a high-frequency signal in a high-frequency circuit or module in which a plurality of electronic components such as semiconductor ICs or chip components are integrated.
2. Description of the Related Art
Conventionally, a wiring board is known which includes a triplate line as means for transmitting a high-frequency signal in a high-frequency circuit or module in which a plurality of electronic components such as semiconductor ICs or chip components are integrated.
FIG. 10 is a cross-sectional view of the structure of a conventional wiring board including the triplate line. The conventional wiring board 500 includes a first grounded conductor 510, a second grounded conductor 520, a dielectric material layer 530 provided between the first and second grounded conductors 510 and 520, and a signal line 540 embedded in the dielectric material layer 530. A cross-sectional shape of the signal line 540 is usually rectangular. In the conventional wiring board 500, a thickness ha′ of a first region of the dielectric material layer 530 located between the signal line 540 and the first grounded conductor 510 is equal to a thickness hb′ of a second region of the dielectric material layer 530 located between the signal line 540 and the second grounded conductor 520. This structure is shown in FIG. 6 of Japanese Patent Laid-Open Publication No. 2003-69239, for example. A relative dielectric constant ∈a′ of the first region is equal to a relative dielectric constant ∈b′ of the second region and a capacitance of the first region with respect to the ground is equal to that of the second region.
The triplate line is called as a strip line in some cases. The term “triplate line” herein can be read as a strip line.
The first grounded conductor 510 and the second grounded conductor 520 may function as capacitive elements in a parallel plate comb structure formed by a first signal line and a second signal line, the first signal line being implemented by the first grounded conductor 510 and the second grounded conductor 520 not grounded and maintained at the same potential, and the second signal line being the signal line 540.
Further reduction in size and lower transmission loss are required in a high-frequency module. In order to achieve those, the wiring board including the triplate line has the following problems. The same problems are encountered in a wiring board with capacitive elements.
Characteristic impedance (Z0) of the triplate line is represented by the expression (1).
                              Z          0                =                              L                          C              T                                                          (        1        )            In the expression (1), CT (F/m) is a capacitance of the triplate line and represents a capacitance of the signal line with respect to the ground. Taking the conventional wiring board 500 shown in FIG. 10 for example, the capacitance CT with respect to the ground is determined by a sum of an electrostatic capacitance Ca between the signal line 540 and the first grounded conductor 510 and an electrostatic capacitance Cb between the signal line 540 and the second grounded conductor 520. Moreover, L (H/m) is an inductance of the triplate line.
As is apparent from the expression (1), the characteristic impedance (Z0) is in inverse proportion to a square root of the capacitance CT with respect to the ground. The capacitance CT with respect to the ground is in proportion to the width of the signal line. Thus, when a size of the signal line is reduced in the conventional wiring board, the capacitance CT with respect to the ground is reduced and accordingly the characteristic impedance (Z0) increases. In other words, it is difficult to make the signal line narrower in the conventional wiring board because it is necessary to keep the characteristic impedance (Z0) the same. This makes it difficult to increase density of the high-frequency module.
Next, the problem in reducing the transmission loss in the wiring board will be described.
The transmission loss in the wiring board is determined by a sum of a dielectric loss and a conductor loss.
The dielectric loss is in proportion to a frequency f of a high-frequency signal transmitted in the signal line, a relative dielectric constant ∈r of the dielectric material layer, and dielectric tangent tan δ of the dielectric material layer. Thus, the dielectric loss is determined by characteristics intrinsic to an insulating material used for the dielectric material layer under a condition where the frequency f of the high-frequency signal is constant.
On the other hand, the conductor loss is in proportion to a square root of the relative dielectric constant ∈r, a square root of the frequency f of the high-frequency signal, and a square root of a specific resistance ρ of the signal line.
A skin effect that is related to current density of the signal line will now be described. When a high-frequency signal is transmitted in the signal line, a back electromotive force disturbs a current flow around the center of the signal line and the current density concentrates on the surface of the conductor of the signal line. This phenomenon is called as a skin effect and the depth of the current flow is called as a skin depth δ. The skin depth δ is represented by the expression (2).
                    δ        =                                            2                              ω                ⁢                                                                  ⁢                μ                ⁢                                                                  ⁢                σ                                              =                                    1                              π                ⁢                                                                  ⁢                f                ⁢                                                                  ⁢                μ                ⁢                                                                  ⁢                σ                                                                        (        2        )            
In the expression (2), ω is an angular frequency (rad/s), f is a frequency (Hz), μ is a magnetic permeability of the signal line, and σ is an electric conductivity (S/m).
As is apparent from the expression (2), the current concentrates on the surface of the signal line more and the specific resistance ρ becomes larger apparently, as the frequency f becomes higher. The increase of the specific resistance ρ leads to increase in the conductor loss. Moreover, surface roughness of the signal line becomes a factor of the conductor loss caused by an eddy current. As the skin depth δ becomes smaller, the current is affected by the surface roughness more easily and the conductor loss increases.
The skin effect tends to become pronounced at an edge of the signal line. Thus, in the case where the cross-sectional shape of the signal line is rectangular as in the conventional wiring board, the current density concentrates on each corner.
The conductor loss is in proportion to the square root of the frequency f, too, as described above. Thus, as the frequency f becomes higher, the conductor loss rapidly increases. In this manner, the conductor loss caused by concentration of the current density of the signal line on the surface becomes a problem in the conventional wiring board, especially when the frequency is high.